Nitrile polymers and copolymers, and especially nitrile rubber, are widely employed in applications in which fuel and oil resistance are desirable, or in which heat resistance is desired. Such polymers and copolymers are well-known, however, for their poor weathering properties. Particularly, these polymers are known to display surface cracking as a result of exposure to ozone rich atmospheres. Poor weathering resistance has limited the use of such polymers and copolymers in applications such as belts, hoses, hose covers, wire covers, and other applications in which the compounded and cured nitrile polymer is exposed to a hostile atmosphere.
Unhydrogenated nitrile copolymers having a high acrylonitrile content (45-55% by weight acrylonitrile) provide low fuel and oil permeability, and slightly better weather resistance than lower acrylonitrile copolymers, but exhibit poor processability. Such copolymers also tend to exhibit high temperature properties characteristic of nitrile rubbers.
Hydrogenated nitrile copolymers, on the other hand, are known to have a longer high temperature service life than their unhydrogenated counterparts, and those that are fully (100%) saturated (or hydrogenated) exhibit good weather resistance. Such compositions are prohibitively expensive for many applications in which high service temperatures are required, since hydrogenation typically employs expensive and unrecoverable catalyst systems.
In order to overcome weatherability problems, such as ozone resistance, PVC has been blended with nitrile polymers and copolymers to obtain ozone protection. Though providing ozone protection, thermoplastic PVC imparts poor heat resistance to the blended polymer. This has the adverse effect of limiting the heat resistance, low temperature properties, and flexibility of the blended polymer, since the thermoplastic characteristics of the PVC are found in the resulting blended polymer.
Others have attempted to obtain improved heat resistance by blending mineral fillers, or by employing CdO-sulfur donor or peroxide cure systems, in the production of nitrile rubber. These systems have, however, provided only slight improvement in the service life of nitrile polymers and copolymers at high temperatures, but they do not improve weatherability.
Another solution proposed to overcome these problems is combining hydrogenated nitrile rubber with a low molecular weight liquid copolymer of acrylonitrile and butadiene. For instance in U.S. Pat. No. 4,647,627, a mixture or covulcanizate of acrylonitrile-butadiene copolymers is prepared from 90-55 percent by weight of 18-49 percent conventional type acrylonitrile copolymer (Mooney viscosity ML 1+4/100.degree. C. of 25 ME minimum), and from 10-45 percent by weight partially or completely hydrogenated liquid copolymer having an average molecular weight of 0.5-30 kg/mole and an acrylonitrile content of 15-48 percent by weight. Because at least one liquid copolymer is employed, the mixture or covulcanizate produced has a low Mooney viscosity; as shown in Example 3 of the patent, ML 1+4/100.degree. C.=33 ME. A nitrile rubber composition so produced would therefore find little utility in applications in which both ozone protection and heat stability are desirable, since it would exhibit poor processability and inferior physical properties, particularly heat stability.
Similarly, in U.S. Pat. No. 4,421,884, a low molecular weight liquid terpolymer of an unsaturated nitrile, a conjugated diene, and optionally a vinyl monomer are combined with a partially hydrogenated unsaturated nitrile/conjugated diene copolymer in specified quantities. As above, the rubbers so produced are recognized to have low Mooney viscosities, resulting in poor processability and physical properties, particularly heat resistance, among other problems. Furthermore, the addition of a vinyl monomer to the terpolymer adversely affects the heat resistance of the rubber so produced.
Thus, none of the systems previously proposed exhibit the desired combination of producing a polymer system offering heat resistance, weather resistance, low fuel and oil permeability, good processability, and relatively low cost required for many applications in which nitrile polymers are employed.